A downhole steering tool (or “controllable stabiliser”) is described in EP 1 024 245. As indicated in that document, the steering tool is used to control the drilling direction by forcing a part of the driveshaft away from the longitudinal centreline of the borehole, thereby forcing the drill bit to deviate from a linear path.
The steering tool comprises a number of steering pistons located in respective steering cylinders spaced around the circumference of the steering tool, the steering cylinders being individually pressurised whereby the steering pistons to opposed sides of the steering tool can project from their steering cylinders by differing and controllable distances. The pressure of the fluid within each steering cylinder, and therefore the projection of each steering piston, is controlled by a commutating valve which delivers hydraulic fluid to each of the steering cylinders in turn, the pressure delivered to each steering cylinder being determined in accordance with the desired steering piston projection.
The present invention relates to a valve which is designed primarily to replace the commutating valve described in EP 1 024 245 The teaching of that document is incorporated into this document by reference so as to avoid the unnecessary repetition of much of the common componentry and method of operation. Whilst much of the following description therefore relates to the invention incorporated as a commutating valve in a downhole steering tool, it will be understood that the invention could be used on other downhole applications.
FIG. 4 of EP 1 024 245 is reproduced as FIG. 1 herein for ease of reference. The steering tool (16) surrounds a part of the driveshaft (12). The driveshaft (12) is connected to the drill string and the drill bit, neither of which is shown in FIG. 1, in known fashion. The steering tool (16) comprises a body (48) and a sleeve (58), the body (48) and sleeve (58) being separated by a fluid-filled annulus (56). Formed in the body (48) are six steering cylinders (50), each of which carries a steering piston (52), only one of the steering cylinders and steering pistons being shown in FIG. 1 (the other five steering cylinders and steering pistons are identical to the steering cylinder and steering piston shown in FIG. 1, and spaced at 60° intervals around the body (48)). In use, the driveshaft (12) will rotate and the body (48) will be substantially non-rotating, i.e. the body (48) will be held substantially rotationally stationary by the engagement of the sleeve (58) with the surrounding borehole wall. Whilst an arrangement having six steering cylinders (50) is described in EP 1 024 245, practical embodiments of that invention have twelve steering cylinders spaced at 30° intervals around the body. It will be understood that the present invention can be used with any number of steering cylinders.
The steering tool (16) has an annular reservoir of pressurised hydraulic fluid which is not seen in FIG. 1, the reservoir being connected to a channel (26) located in an annular commutating valve (24). Importantly, the commutating valve (24) is fixed to rotate with the driveshaft (12) and therefore rotates relative to the body (48), so that as the driveshaft (12) and commutating valve (24) rotate the channel (26) periodically communicates with each of the steering cylinders (50) by way of its respective conduit (54).
A solenoid valve (not seen in FIG. 1) can be opened to relieve the pressure within the channel (26). As long as the solenoid valve remains open all of the steering cylinders (50) experience the same pressure and the body (48) (and therefore the driveshaft (12)) remains centrally located relative to the sleeve (58) and borehole. To cause the driveshaft (12) to move away from its central location the solenoid valve is closed whilst the channel (26) is in communication with a chosen steering cylinder (50), that steering cylinder receiving higher-pressure hydraulic fluid which causes the respective piston (52) to be driven outwardly.
It is essential to the correct operation of the commutating valve (24) that the higher-pressure hydraulic fluid is only delivered to the chosen steering cylinder (or chosen adjacent steering cylinders), and this requires a fluid-tight seal to be present between the commutating valve (24) and the body (48). The seal between these components, both in FIG. 1 and in practical embodiments of the steering tool, is provided by the accurate machining of the sliding metal surfaces.
In practice, the steering tool (16) experiences significant temperature variations in use, and the thermal expansion of the commutating valve (24) and body (48) prevent the maintenance of a perfect seal. In practical embodiments the adverse effects of the less than perfect seal are sought to be overcome by using a more viscous hydraulic fluid. However, there is a limit to the viscosity which can be used since the solenoid valve must operate with the hydraulic fluid, and if the hydraulic fluid is too viscous the solenoid valve will not be able to close. Whilst a spring can be used to assist closure of the solenoid valve the force provided by the spring must be somewhat less than the electromotive (valve opening) force which can be provided by the solenoid, so that in practice a strong spring cannot be used to assist valve closure.
Also, a given steering tool is likely to have to operate in different temperature regimes, i.e. the temperature varies according to the depth and location of the borehole in which the steering tool is being used, and a high viscosity hydraulic fluid suitable for use in a high temperature borehole might be too viscous in a low temperature borehole, resulting in significant pumping losses and perhaps leading to tool failure in the event that the solenoid valve cannot close. Alternatively, a low viscosity hydraulic fluid suitable for a low temperature borehole is likely to leak between the commutating valve (24) and the body (48) when used in a high temperature borehole, again perhaps leading to tool failure.